Polymer-based composites filled with inorganic particulates have long been used for improving mechanical properties of thermoplastic and thermoset polymers. Composites loaded with moisture absorbers have been also proposed for the purpose of making desiccating liners and other internal parts of thermoplastic containers and controlling humidity in such containers, e.g., U.S. Pat. No. 6,130,263. Multiple electronic, automotive, pharmaceutical, diagnostic, and food packaging applications require very low humidity levels inside the package or encapsulated enclosure to protect moisture-sensitive products or components from exposure to environmental humidity. Increasingly, electronics and automotive electronics are shifting from the use of metal die castings for housings and components to the use of thermoplastics. This transition is fueled by the need to reduce weight and cost in an ever increasing competitive environment. Resulting from the rapid development pace of these applications, the selection process of thermoplastic resins often does not give consideration to the water vapor transport properties of such resins which directly impact the service life of the components or device. In conjunction with this trend is the miniaturization of components or devices which leaves little or no space for enclosed sorbents which results in inadequate or no moisture protection whatsoever. Currently, the selection and use of thermoplastics exhibiting the best passive moisture barrier characteristics often falls short of providing protection from moisture ingress over the desired service life of the component or device. Heretofore, enclosures of this type have essentially failed to provide a sufficient performance regarding prevention of moisture ingress.
Several U.S. patents focus on improving moisture transport into the desiccant-loaded composite in order to accelerate the desiccating effect of such composites by rapidly removing moisture from inside the package, especially when used as an interior part of the container. See, for example, U.S. Pat. Nos. 5,911,937; 6,130,263; 6,194,079; and, 6,214,255. The devices are inadequate for many described needs as they fail to prevent moisture ingress, but merely absorb moisture after it has entered an enclosure during packaging operations, during service time of a device or after opening of resealable containers.
Since all plastic materials are permeable to atmospheric gases and water vapor, plastics are unable to provide a moisture-free environment inside any package or enclosure made therefrom, for significant periods of time. Heretofore, controlling permeated moisture, i.e., moisture which has already entered an enclosure, has been the primary motivation for developing moisture-absorbing composites. An alternative approach to removing moisture that has already permeated into a package is to improve the moisture barrier properties of the packaging polymeric material itself. Dispersing a water absorber in a thermoplastic polymer resin allows the creation of a composite that acts as an active barrier to water vapor permeation. Active barriers intercept and remove moisture diffusing across the barrier via physical and/or chemical means. Such barriers allow for the reduction or complete elimination of moisture permeation into the package for significant time durations. See, for example, U.S. patent application Ser. Nos. 11/040,471; 11/335,108; and 11/635,750, which applications are incorporated herein by reference in their entirety.
In composite barriers filled with moisture adsorbing additives, the sorbent additive distributed in the polymeric matrix can cause a significant delay, known in the art as a diffusive lag time of a barrier layer, before moisture starts to penetrate into a package, which is often several orders of magnitude longer than the delay observed in barriers made from neat resins. Although the calculation of the lag time for neat resins and resin/sorbent composite barriers is described in the DETAILED DESCRIPTION OF THE INVENTION below, it should be appreciated that these calculations are well known in the art. These lag time calculations have been used as a determinant of enclosure failure. In other words, once the lag time is reached, the enclosure is assumed to have failed since unimpeded moisture permeation across the barrier is presumed to proceed thereafter. Contrarily, it has been found that this assumption is incorrect for certain types of sorbent materials and barrier compositions. For example, for a period of time after reaching the lag time, an enclosure may be maintained below a relative humidity of 10-50%, and provided that the contents of the enclosure are not deteriorated and/or damaged by such relative humidity levels, the enclosure has not failed by merely reaching the lag time. Thus, it is advantageous to understand how an enclosure performs after reaching the lag time.
As can be derived from the variety of devices and methods directed at producing moisture adsorbing polymers used for either providing an active barrier or for absorbing existing moisture from within an enclosure, many means have been contemplated to accomplish the desired end, i.e., protection of a moisture sensitive component from degradation, thereby prolonging the service life of the component. Heretofore, tradeoffs between material selection and mix ratios were required, and as described, such determinations were made without fully understanding their effects. Thus, there is a long-felt need for a method of selecting the polymer resins, sorbents and mixtures ratios thereof to achieve specific design and performance targets. There is a further long-felt need for a means to accurately predict the effects of polymer resin, sorbent and mixture ratio thereof selections. In view of the foregoing, the present invention permits improving moisture barrier properties of packaging articles rather than merely understanding the rate of moisture sorption therein. Furthermore, the present invention moisture barrier provides effective barrier characteristics at the point of moisture ingress thereby preventing or minimizing moisture permeation, rather than providing for the removal of moisture after it reaches the interior of an enclosure.